Hysteresis and thermomagnetic properties of particle-sized fractions from loess and palaeosol samples spanning 22 Myr of accumulation on the Chinese Loess Plateau

Abstract

Hysteresis loop measurements and thermomagnetic experiments were carried out to characterize magnetic mineralogy and magnetic grain size in a suite of particle-size fractioned samples from the Chinese Loess Plateau, representing the main changes in magnetic properties over the last 22 Myr. We find that (1) the ferrimagnets in the detrital fractions (>8 μm) younger than 14 Ma show a magnetite-type Curie temperature, whereas those older than 15 Ma have a Curie temperature slightly over 600 °C owing to low-temperature oxidation. (2) The dominant ferrimagnets in the clay fractions (<2 μm) are most likely to be maghemite. (3) Both the hysteresis plots and thermomagnetic experiments reinforce previous indications from low-temperature experiments that paramagnetic minerals make a major contribution to the clay fractions in all the samples considered. (4) The degree of oxidation indicated by the thermomagnetic curves is highest in the clays and next highest in the oldest coarse fractions. (5) Day plots provide limited and potentially misleading information on the differences between fine-grained pedogenic and coarse-grained detrital ferrimagnetic assemblages, even when allowance is made for the effects of hematite on the quotients used. This study reinforces the view that measurements on bulk samples alone obscure critical distinctions between pedogenic and detrital components. © 2012 The Authors Geophysical Journal International © 2012 RAS.